1. Field of the Invention
The present invention relates to an apparatus for detecting a wheel grip on a road, i.e., a condition of wheel contact with the ground, in order to detect so-called hydroplaning or aquaplaning phenomena which occur when a vehicle is running at a high speed on a road surface which is wet with water.
2. Description of Related Art
The hydroplaning phenomena can be classified into a complete hydroplaning in which wheels of the vehicle are completely running on a film of water and a partial hydroplaning in which, though the wheels of the vehicle are lifted or suspended by the film of water, the wheels are still partially maintaining contact with the road surface.
Once the complete hydroplaning phenomena have occurred, the wheels come to be subjected to a strong resistance from the film of water. It is known, then, to detect the occurrence of the hydroplaning phenomena based on differential changes in a revolution speed of the wheels of the vehicle.
In the partial hydroplaning, however, the resistance that the wheels receive from the film of water is not very large. Therefore, it is difficult to detect the occurrence of the partial hydroplaning by the conventional method.
As a result of diligent studies, the inventors of the present invention have found out that, once the partial hydroplaning has occurred, those frequency components of higher harmonics which are contained in an output signal of a wheel speed sensor for detecting a revolution speed of a wheel deviate by several Hz towards the low-frequency side by receiving a viscous resistance due to the film of water.
The present invention has been made based on this finding, and has an object of providing an apparatus that can detect the occurrence of the partial hydroplaning.